1. Field of the Invention
This invention relates to high-speed chemical reactions and, more particularly, to a novel apparatus and method for time-domain tracking of high-speed chemical reactions, and specifically for thermal processing of oil shale using microwave heating of the oil shale.
2. The Prior Art
The quantity of oil shale in the world represents a very large energy resource. One estimate states that there is a total resource of oil shale in the United States of about 2.2 trillion barrels of which about 80 billion barrels are considered as recoverable reserves using existing technology. As with other energy sources, however, the estimates of the magnitude vary widely.
The term "oil shale", although a misnomer, is a term used to refer to a marlstone deposit interspersed with inclusions of a solid, coal-like organic or hydrocarbon polymer referred to as "kerogen". Kerogen is a macromolecular material having a molecular weight greater than 3,000 with an empirical formula approximating C.sub.200 H.sub.300 SN.sub.5 O.sub.11. The composition of the organic material from oil shale taken from the Mahogany zone of Colorado revealed a carbon content of approximately 80.5 percent by weight with 10.3 percent hydrogen, 2.4 percent nitrogen, 1.0 percent sulfur, and 5.8 percent oxygen for a carbon/hydrogen ratio of about 7.8. It should be noted that the carbon/hydrogen ratio for petroleum ranges between 6.2 and 7.5. Kerogen predominantly has a linearly condensed, saturated cyclic structure with heteroatoms of oxygen, nitrogen, and sulfur with straight-chain and aromatic structures forming a minor part of the total kerogen structure. Synthetic liquid and gaseous products that have some similarities to oil or oil products can be extracted from the kerogen, although the products are not a true oil product. Different solvents and different degradation temperatures yield products with different compositions.
Over the years, various in situ processes have been suggested to recover useful fuels from oil shale deposits. These processes generally involve conventional thermal processes which require development of a thermal gradient; that is, the outside of the shell block being maintained at a higher temperature than the inner portion. However, large thermal gradients represent an inefficient use of the applied thermal energy, and can also lead to a degraded shale oil product having a very high pour point.
When oil shale is heated to about 430.degree.-480.degree. C., the kerogen decomposes to form oil, gas, bitumen, and a carbonaceous residue which is retained on the spent shale. The bitumen decomposes further to form oil, gas, and additional residual carbon. Because of the very complex nature of kerogen, various reaction mechanisms have been proposed. However, the reaction has generally been treated as though it were first order with respect to the concentration of kerogen in the formation of bitumen and also first order with respect to bitumen decomposition in the subsequent formation of oil and gas. While the resultant oil and gas product migrates to the surface of the shale and is swept away, the residual carbon remains on the spent shale.
Residual carbon is an energy source that can be utilized by conventional combustion techniques to provide thermal energy for the process. In situ combustion of this residual carbon for the production of products from oil shale involves the regulated introduction of oxygen into a previously rubbilized oil shale formation for the purpose of controlling combustion of the residual carbon. However, when the size of the oil shale formation is sufficiently large, as in most in situ retorting processes, the residual carbon or char is not completely burned, thus necessitating combustion of a portion of the product oil vapor to supplement the required thermal energy. Additionally, direct combustion of carbonaceous residue takes place in proximity to the zone where the oil vapor is being produced thereby increasing the probability that oxygen will reach the latter zone and oxidize a portion of the oil vapor. This problem is more severe in in situ combustion retorting processes in which oil shale blocks of wide size distribution are retorted.
The flow of gases in large oil shale blocks is much more nonuniform which, in turn, increases the infiltration of oxygen into the zone of oil vapor production. Furthermore, it has also been found that an attempt to increase the retorting rate is generally accompanied by a corresponding increase in the combustion rate of the oil vapor thereby further lowering the product recovery ratio.
Another traditional approach for extracting kerogen or, more precisely, products therefrom, from oil shale is to heat the oil shale in an above-ground retort. The oil shale is mined and then processed by size reduction for ease of handling and good thermal (gas/solid) transfer. While the extraction of kerogen from the inorganic, mineral matrix is highly efficient in an above-ground process, an underground mining operation leaves about 35 percent of the oil shale in place for structural support in the mine. Furthermore, a mining operation followed by an above-ground thermal processing is economically viable only with the very high grade oil shale materials (generally greater than about 25 gallons per ton).
The use of radio frequency (RF) dielectric heating represents a new and alternative technology to recover useful fuels from oil shale and other hydrocarbonaceous deposits. By this method, large blocks of oil shale can be heated from within to a uniform temperature. This heating is independent of the thermal conductivity and gas permeability of the raw oil shale. Additionally, RF heating can result in a nearly true in situ process because only one to three percent of the oil shale is removed to place electrodes thereby allowing a large percentage of the deposit to be processed. Environmental problems are also minimized (1) by leaving the spent shale in place and (2) by avoiding in-place combustion.
One useful publication relating to the dielectric heating of oil shales is Comparison of Dielectric Heating and Pyrolysis of Eastern and Western Oil Shales, R. H. Snow, J. E. Bridges, S. K. Goyal, and A. Taflove, IIT Research Institute, 10 West 35th Street, Chicago, Ill. 60616.
However, another study found that the amount of RF energy absorbed by the oil shale was so small that reflected energy was nearly the same as the incident energy. Additionally, it was found that the results were both void-fraction-dependent and frequency-dependent. The ultimate conclusion from this latter study was that the frequency dependence was not regarded as having practical significance since development reactors will most likely be designed around a battery of cheap and available 2450 MHz magnetron tubes, the kind of tube used in the study. The conclusion drawn from this latter study was that the most relevant outcome was the discovery that oil shales vary in unexpected ways in their RF absorption characteristics. It was therefore assumed that if an RF processing technique should prove to be worthy of development, very careful analysis of the oil shales would be necessary. See Study of the Chemical Values of Oil Shale Through Rapid Pyrolysis, N. W. Ryan, pg. 187 of Final Report on Selected Research Projects Leading to the Development of Utah Coal, Tar Sands, and Oil Shale, College of Mines and Mineral Industries, College of Engineering, and the Utah Engineering Experiment Station, October 1978.
However, it is also important to note that the careful analysis of oil shales during rapid heating is extremely complicated since the chemical changes occurring during rapid heating are extremely fast or even abrupt, thus prohibiting a careful analysis of these changes using conventional techniques.
In view of the foregoing, it would be a significant advancement in the art to provide a novel apparatus and method for tracking high-speed chemical reactions. It would also be an advancement in the art to provide a novel apparatus and method for tracking the high-speed or abrupt thermal decomposition of kerogen in oil shales upon heating by RF dielectric heating. It would also be an advancement in the art to provide a novel apparatus and method for tracking changes in the permittivity of oil shales. It would also be an advancement in the art to provide a novel process for heating kerogen in oil shale using RF dielectric heating while maintaining the optimum RF frequency for heating. Another advancement in the art would be to provide a feedback system to adjust the frequency of the RF radiation to consistently correspond to the relaxation frequency required for optimum RF heating. Such a novel apparatus and method is disclosed and claimed herein.